I just want to clarify for perhaps non-horsey people reading this thread - the lethal gene in horses being talked about is linked to FRAME overo, specifically. Not other common overo patterns like splash or sabino. It is known as Overo Lethal White syndrome (OLWS) or Lethal White Overo (LWO), which isn't an entirely accurate name because it is the frame overo gene that is lethal.

(eta: it is lethal due to an abnormal and dysfunctioning colon, IIRC. There is no treatment or cure that I have heard of - foals must be PTS or die naturally. They appear healthy and normal otherwise. Interestingly, Paint breeders report a far fewer percentage of lethal white foals than the 25% it "should" be when breeding frame to frame. More like 4-5%, I've read in some cases.)